1. Field of the Invention
This invention relates to rubber compositions having short fibers dispersed therein. The invention is also directed to power transmission belts made with the rubber composition and to a method for making the rubber composition and power transmission belts.
2. Background Art
The number of uses for rubber parts and the demands on these parts, particularly in the automotive industry, have increased over the years. Designers of rubber products, including power transmission belts, have striven to make these products capable of surviving in severe environments while meeting demanding performance criteria.
The nature of rubber products is generally determined by the raw material rubber and particular compounding ingredients used with the rubber. Recently, there has been a trend towards dispersing short fibers in the rubber to enhance the properties thereof, namely reinforcing properties, abrasion resistance, etc.
In the automotive industry, power transmission belts are now utilized on engines to convey power to and from different auxiliary equipment, such as air compressors, alternators, etc. By embedding short fibers, such as those made from cotton, nylon, Vinylon, rayon, aramid, etc., in a widthwise orientation in rib portions, side pressure resistance of the driving/driven portions of the belt can be enhanced. Further, by projecting the short fibers from the exposed side surfaces of the belt, desired frictional characteristics may be selected. Additionally, the projecting fibers may be incorporated so as to control sound generation due to sticking between the belts and cooperating pulleys in use.
Belt durability may be improved, particularly by enhancing abrasion resistance at the drive/driven surfaces, by incorporating short aramid fibers and projecting these fibers from the side surfaces of the belt which contact cooperating pulleys in use. Japanese Patent Laid-Open No. 164839/1989 discloses power transmission belts of this type with the incorporation of aramid fibers for enhancing belt durability. In that document, the aramid fibers are described to project to the side wall surfaces on the compression rubber layer of the belt at locations which contact cooperating drive/driven pulley surfaces. However, when the aramid fibers projecting from the belt surfaces are broken off, rigid, aramid fiber portions may remain exposed. During operation, these rigid fiber portions may generate a jarring rubbing sound as they contact cooperating pulleys.
Power transmission belts in the automotive industry are commonly subjected to severe operating conditions. This is particularly true of belts used in engines with speed changing systems. Belts of this type are commonly incorporated into engines having a large capacity. Further, the conditions are often made even more severe by current designs which cater to social demands for energy savings and compacting of engine compartments. Belts in this environment are required to exhibit excellent abrasion resistance, compression resistance, and resistance to flexing fatigue. Often, in these severe environments, even aramid fibers, which are recognized to have excellent abrasion resistance, may be inadequate to meet operating demands.
The power transmission belt industry is constantly looking for belt designs capable of effectively operating for long periods under the extreme conditions encountered in the automotive industry, and elsewhere.
It is known to treat short reinforcing fibers dispersed in rubber to improve characteristics of a rubber composition. Japanese Patent Publication No. 24131/1985 discloses a method of treating fibers using an RFL liquid having a carboxyl group-containing acrylonitrile-butadiene rubber latex. Japanese Patent Publication Nos. 41525/1993, 41526/1993, and 41527/1993 disclose methods of treating short fibers with an adhesive composition containing RFL liquid, a halogen-containing polymer, and an active compound selected from an isocyanate compound, an epoxy compound, and a silane coupling agent, as main constituents. Japanese Patent Publication No. 41528/1994 discloses a method whereby short fibers are first treated with an epoxy compound or an isocyanate compound, thereafter treated with RFL liquid, and thereafter further treated with a rubber paste that is made by dissolving a rubber compound and chlorinated rubber in a solvent.
However, using the above treating techniques with poly(paraphenylene benzobisoxazole) short fibers (PBO short fibers) has been found by the inventors herein not to produce the rubber composition characteristics required in certain operating environments. Further, the PBO short fibers, so treated, may not have desired dispersing qualities. If the dispersion of the fibers is not uniform, the rubber composition is prone to cracking. This problem results from the fact that the PBO fibers, treated in the above manner, by reason of their molecular structure, generally do not have the degree of adhesion with rubber as do other commonly used fibers. Further, by requiring the post-treatment use of RFL liquid, increased labor, time, and cost may be associated with manufacture.